ABSTRACT

Confined geometries offer useful and experimentally amenable mechanical testing arrangements in which to study the molecular and micro-structural processes which govern plastic yield in stress environments dominated by hydrostatic pressure over shear. However, the changes to macroscopic stress – strain behaviour that result from switching from an unconfined mode such as uniaxial compression to a confined one are often overlooked and display a surprising level of complexity, even for simple elastic – plastic constitutive models. Here we report a confinement induced strain hardening effect in polystyrene thin films achieved through repeated plastic loading with a cylindrical flat punch whose diameter is many times the initial film thickness. This high aspect ratio combines with constraint provided by film material surrounding the contact to generate a state of confined uniaxial strain in the indented region, rendering the deformation one dimensional. By repeated loading past the yield point into the plastic domain, we achieve a 66% increase in the confined yield stress, from 0.3 GPa to 0.5 GPa. Through finite element simulation and analytic modelling of the principal stresses and strains, we show that this effect arises not from intrinsic changes to the structure of the material, but rather residual stresses imparted during plastic loading. We contrast this effect with intrinsic changes to glassy thin films such as physical ageing and thermal cross-linking.

摘要

受限的几何形状提供了有用的并且在实验上适合的机械测试装置，其中可以研究在剪切流体静压力为主导的应力环境中控制塑性屈服的分子和微观结构过程。但是，由无限制模式（如单轴压缩）转换为有限模式而引起的宏观应力－应变行为的变化通常被忽略，即使对于简单的弹性－塑料本构模型，也显示出令人惊讶的复杂性。在这里，我们报告了通过用圆柱形平冲头反复进行塑料加载来实现的聚苯乙烯薄膜中的局限诱导应变硬化效果，该圆柱形平冲头的直径是初始薄膜厚度的许多倍。这种高的长宽比与围绕触点的薄膜材料所提供的约束相结合，从而在凹入区域中产生受限的单轴应变状态，从而使变形为一维。通过反复将超过屈服点的载荷加载到塑性区中，我们将受限屈服应力从0.3 GPa增加到0.5 GPa达到66％。通过有限元模拟和主应力和应变的解析模型，我们证明了这种影响并非源于材料结构的内在变化，而是源于塑料加载过程中产生的残余应力。我们将此效果与玻璃薄膜的固有变化（例如物理老化和热交联）进行对比。通过反复将超过屈服点的载荷加载到塑性区中，我们将受限屈服应力从0.3 GPa增加到0.5 GPa达到66％。通过有限元模拟和主应力和应变的解析模型，我们证明了这种影响并非源于材料结构的内在变化，而是源于塑料加载过程中产生的残余应力。我们将此效果与玻璃薄膜的固有变化（例如物理老化和热交联）进行对比。通过反复将超过屈服点的载荷加载到塑性区中，我们将受限屈服应力从0.3 GPa增加到0.5 GPa达到66％。通过有限元模拟和主应力和应变的解析模型，我们证明了这种影响并非源于材料结构的内在变化，而是源于塑料加载过程中产生的残余应力。我们将此效果与玻璃薄膜的固有变化（例如物理老化和热交联）进行对比。而是在塑料加载过程中施加的残余应力。我们将此效果与玻璃薄膜的固有变化（例如物理老化和热交联）进行对比。而是在塑料加载过程中施加的残余应力。我们将此效果与玻璃薄膜的固有变化（例如物理老化和热交联）进行对比。